1. Field of the Invention
The present invention relates generally to the conversion of electric signals from analog to digital form. More specifically, the present invention relates to a digital circuit for translating an analog video or like analog signal into an equivalent digital signal with the addition of dither which is an artificially created noise signal and which provides for a reduction of noise and distortion in the signal.
2. Description of the Prior Art
The pulse code modulation (PCM) or digital processing of analog audio signals has become, or is becoming, the mainstream of high fidelity sound recording and reproduction with the advent and ever increasing commercial acceptance of compact discs. The digital processing of analog signals in the low to high frequency range is also becoming highly useful for analyzing the performance of weapons systems such as guided missiles. For example, the analog data acquired by a guided missile during flight is converted to its digital equivalent and then placed in a digital PCM frame for transmission to a ground station for analysis.
There are, however, some problems left unsolved in the art of digital processing of analog signals. For example, there is a significant difference between the samples of an analog signal and the quantized values of the samples which is generally referred to as "quantization noise". This difference becomes particularly pronounced when the input analog signal is low and there are only a small number of quantization steps, thereby, for example, distorting reproduced sound as higher harmonics in high fidelity sound systems. Even when an input analog audio or like signal level is high, the quantization noise will distort the signal if it changes slowly.
A prior art solution to this quantization noise problem has been the use of dither which is intended to turn the quantization noise into broad band white noise which has little or no effect on reproduced sound or like analog signals and which is generally referred to as "white noise". An analog dither signal is superposed on the analog audio or like signal prior to its digitization and may, or may not be removed from the digitized audio signal.
One system of the prior art adds an analog dither signal, converted from the output from a digital dither generator by a digital to analog converter, to the incoming analog audio or analog data signal. The combined analog data and dither is digitized by an analog to digital converter and then fed to a subtractor circuit. The subtractor circuit is also supplied the digital dither directly from the dither generator. The subtractor circuit then subtracts the dither component from the digital data and dither signal.
A second system of the prior art uses an analog dither generator in combination with a second analog to digital converter for digitizing the analog dither prior to its delivery to the subtractor. In this prior art system, analog dither is added directly to the analog data signal. The resulting dither added data signal is then digitized by the first analog to digital converter and the analog dither is digitized by the second analog to digital converter preparatory to application to the two inputs of the subtractor circuit.
During the conversion process from an analog signal to its digital equivalent, prior art systems often use from as little as three digital bits to one digital bit to represent the incoming analog signal. However, in certain applications, such as data received from a missile during flight, there is a need to increase the resolution of the equivalent digital signal provided by the missile's telemetry system. This necessitates the use of, for example, fourteen bit equivalent digital words to represent the incoming analog signal. This increase in the number of bits to represent the incoming analog signal results in a significant decrease in the error being introduced in the higher resolution bits (least significant bits) and a significant increase in the bandwidth required for transmission of the digital equivalent signal provided by the missile's telemetry system. By utilizing a form of automatic gain control (AGC) after the incoming analog signal is converted to its digital fourteen bit equivalent, a subset, for example, of eight of the fourteen bits may be transmitted with a significant increase in the accuracy and resolution of the transmitted digital information within a reduced bandwidth.
In view of the foregoing, it is an object of the present invention to provide an electronics circuit for accurately converting an analog audio or like data signal into its digital equivalent.
It is a further object of the present invention to provide a reliable yet relatively simplistic and inexpensive electronics circuitry for converting an analog audio or like data signal into its digital equivalent.
It is yet a further object of the present invention to provide electronics circuitry which is capable of operation with high resolution and conversion accuracy.
It is still a further object of the present invention to provide electronics circuitry which is capable of operation with a minimum bandwidth.
The above and other novel features and advantages of the present invention and the manner of realizing them will become more apparent and the invention will be best understood from a study of the following description and appended claims, with reference to the attached drawings.